1. Field of the Invention
This invention relates to a start up fuel control device for an internal combustion engine which is capable of adjusting the amount of fuel supplied to the engine in such a manner that the engine starts in a short time.
2. Description of the Related Art
In some cases, such as a cold start of an engine in a low ambient temperature, a long starting operation (cranking) of the engine is required before the engine starts (i.e. a stable combustion occurs in each cylinder of the engine). In such a case, since a portion of the fuel supplied to the cylinder accumulates therein without being expelled during exhaust strokes, an air-fuel ratio of the air-fuel mixture in the cylinders of the engine gradually becomes lower (i.e., becomes rich) during the starting operation. Therefore, if the starting operation continues for a long time, the air-fuel ratio of the air-fuel mixture in the cylinder may become excessively rich. This makes the engine impossible to start. To prevent this problem, caused by residual fuel in the cylinders, a start up fuel control device is used for performing a scavenging operation of the engine when the cranking of the engine continues, without firing, for more than a predetermined time.
This kind of the start up fuel control device is disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 3-185239. The device in this publication is equipped with means for detecting the occurrence of firing in the respective cylinders of the engine. When firing is not detected after the engine rotates a predetermined stroke cycles (for example, 10 stroke cycles), the device initiates a scavenging operation in which the amount of the fuel supplied to the engine is gradually reduced to expel the residual fuel in the cylinders by cranking. Further, when the initial firing is detected in a cylinder, the device increases the amount of the fuel supplied to the cylinder in which the initial firing is detected until the fuel supply amount reaches the amount before the scavenging operation started. Therefore, the air-fuel ratio of the air-fuel mixture in the non-firing cylinders gradually increases (become less rich) and becomes an air-fuel ratio suitable for firing. Further, if initial firing is detected, the amount of the fuel supplied to the cylinders in which the initial firing is detected is increased to a normal value to thereby obtain stable combustion in these cylinders.
However, sometimes problems arise in the device of the above publication since the device counts the number of the stroke cycles of the engine from the beginning of each starting operation. For example, when the engine fails to start, usually the operator of the engine turns off a main switch of the engine to wait the recovery of the battery voltage and tries to restart the engine. In this case, the device in the above publication clears the number of the stroke cycle in the previous starting operation when the main switch is turned off, and starts counting the cycles from the beginning of the next starting operation. Therefore, if the starting operation is not successful and short starting operations are repeated with the main switch being turned on and off, the device clears the stored number of the cycles every time when the main switch is turned off. Thus, the scavenging operation is not performed in the succeeding starting operation since the number of the cycles in each starting operation does not reach the predetermined value. Therefore, residual fuel in the cylinders increases by repeating the staring operation and the possibility of starting the engine becomes more and more remote.
Further, in the device of the above publication, the scavenging operation of the cylinders continues until initial firing is detected in the cylinders. The air-fuel ratio of the air-fuel mixture in the non-firing cylinders gradually increases (i.e., becomes lean) and passes through the range suitable for firing during the scavenging operation. However, if firing does not occur when the air-fuel ratio passes through the suitable range, the air-fuel ratio in the cylinders becomes excessively lean. Therefore, in the device of the above publication, if firing does not occur when the air-fuel ratio passes through the suitable range, starting the engine becomes impossible due to the excessively lean air-fuel ratio in the cylinders.